The microelectronic technique, the core of which is large scale integrated circuit, is rapidly developed and provides new demands for the micro machining and micro- and nano-detection technique. Since the first commercial projection photolithography machine is developed in American at 1978, the optical projective exposure become a micro machining technique with the widest application, rapid updates and strong vitality, and the core technique to improve the microelectronic technique. Resolution and focal depth of object lens are critical parameters which affect the exposure system. The resolution and focal depth may be calculated based on Rayleigh criterion as follows:k1λ/NA, andDOF=k2λ/NA2.
The resolution for the projection photolithography may be improved mainly by utilizing an exposure light source with more and more smaller wavelength and enhancing a numerical aperture (NA) of the objective lens. At present, exposure wavelength A for the projection photolithography machine is developed from ultraviolet (g-line, i-line) and deep ultraviolet (ArF) towards extreme ultraviolet (EUV), and the NA of the objective lens is developed from 0.2 and 0.8 towards above 1.35 for an immerged projection photolithography. The improvement of the resolution for photolithography is at the cost of focal depth. The focal depth of the projection lens is drastically reduced with the improvement of the resolution for photolithography. Although the resolution may be further improved by utilizing a resolution enhancement technique, it is limited for the improvement of the focal depth. Thus, there is a new demand for the accuracy for adjusting the focal plane of the system by the photolithography machine with a high accuracy.
A measuring sensor for adjusting leveling and focal plane in the photolithography machine usually utilizes an optical sensing technique which has features of speediness, non-contact and so on. Earlier photolithograph machines mostly employ a technique of detecting luminosity and detecting of CCD. By imaging with a slot, a defocusing distance of a silicon wafer may be calculated by computing change of displacement of the slot in a detector. These two detecting methods have a simple measuring system and are easy to be operated; but they have a lower accuracy of measurement and can't meet the requirements of the photolithography on the detection of the focus plane in a higher accuracy. With the enhancement of resolution of the photolithography system and the enlargement of the exposure visual field, the measuring system for adjusting leveling and the focus plane may further utilizes a multipoint measurement in which the height values at multiple points are measured to calculate amounts of inclination of the silicon, which is mainly used by a Japanese company of Nikon. An emerging light of a light source passes through an array of slots, is reflected by a surface of the silicon wafer and is imaged on the detector. When the silicon wafer is in an ideal state, the imaged light spot is located at the center of four-quadrant detector so that the light intensities in the four quadrants. The detection system utilizes the array of slots to cover the whole exposure visual filed by scanning measurements. However, it is necessary to further optimize the algorithm for detection to improve the accuracy of measurement.
With the development of grating detection in the last century, a detection technique based on moiré fringe generated by Talbot effect of a grating is applied to measurements for detecting focus plane in a projection photolithography system. When intervals between two diffraction gratings with comparative cycles satisfy a Talbot distance, a moiré fringe is generated by the two gratings. If the relative positions of the grating and the detector is constant, the signal of the moiré fringe changes due to the change of position of the silicon wafer. Thus, the measurement of defocusing distance of the silicon wafer may be accomplished by measuring information about the change of the moiré fringes. Although such a method has a higher accuracy for detection, the system has a weak anti-interference capability and has a higher requirement for the environment.
A measurement based on gaps of a chirp grating is proposed by Euclid E. Moon et.al. MIT, in 2004 to be applied to measurement for detecting focus plane in the photolithography machine, so as to obtain a detection accuracy in a scale of nanometer. However, since it is limited by graphic processing algorithm and phase analytic algorithm, the system has a poor property in real time and a lower efficiency in detection.
In 2007, researchers in US proposed a method for measuring the focus plane by utilizing aerodynamics principle. A basic idea of this aerodynamic gap measurement is to blast air to a measured surface through a spray nozzle, the change of the gap leads to changes of a return pressure of the spray nozzle and the airflow. Such changes are measured by a pressure sensor or a Flowmeter and then the amount of the gap is derived from it. Due to a higher influence of the environment, there is a report of principle experiment in laboratory, but there is not a success for an actual prototype.
Technique of detecting focus plane in a scale of nanometer based on a spatial light modulation principle is widely used to measurement for detecting the focus plane in a high accuracy for an advanced photolithography machine. The measuring system is constituted of an illuminated light source with a wideband spectrum, a telecentric imaging system and a spatial light modulation and detection system. Such a detection method has a higher accuracy of measurement and can meet the requirement of the photolithography machine in a high accuracy. However, such a system has a complex configuration and employs a detection of point by point scanning, the efficiency of which is lower.
With a development of microfabrication in China, the researchers are focused on the technique for adjusting the focus plane in the photolithography machine. At present, the community for researching the technique for adjusting the focus plane in the photolithography comprises the Institute of Optics and Electronics of the Chinese Academy of Science, Shanghai Micro Electronics Equipment Co, LTD, Huazhong University of Science and Technology, Shanghai Institute of Optics and Fine Mechanics of Chinese Academy of Science and so on.
Among others, the PSD technique is used to measuring for detecting the focus plane by Shanghai Micro Electronics Equipment Co, LTD. The laser beam is collimated, passes through the slots, and is imaged on the silicon wafer. The image is reflected by the surface of the silicon wafer and a mirror and is imaged on the PSD. The whole surface of the silicon wafer is scanned by moving the work table so as to measure the whole silicon wafer. A defocusing distance of the silicon wafer may be calculated by information about the height of the scanned silicon wafer so as to ensure the whole precision for adjusting the focus plane.
Zuohai, YING et. al. at Huazhong University of Science and Technology utilize an array CCD detector to accomplish the measurement for detecting the focus plane in the photolithography machine. Institute of Optics and Electronics of the Chinese Academy of Science is one of the communities which early develops the relevant techniques about the microelectronics equipments and utilizes a method of moiré fringe in a project of 0.8 μm-photolithography machine to obtain a precision of 80 nm for detecting the focus plane.
In general, the currently reported method for detecting the focus plane can't give attention to two or more things of simplicity of the system and the precision of the detection, and can't give attention to two or more things of the precision for detecting the focus plane and the efficiency in a large-area exposure system. For the requirements of higher resolution and higher visual field in the future projection photolithography machines, the present disclosure introduces a method for detecting the focus plane based on Hartmann wavefront detection principle so as to meet the requirements of high precision for detecting the focus plane and the efficiency in the photolithography machines.